1. Field of the Invention
The present invention relates to a nonaqueous secondary battery and to a method of manufacturing a negative electrode active material. More particularly, it relates to a nonaqueous secondary battery where graphite particles in which intercalation and deintercalation of lithium are possible are used in a negative electrode and also to a method of manufacturing a negative electrode active material.
2. Related Arts
With the trends of making the size, the weight and the electric consumption of electronic instruments and the like smaller, lighter and less, respectively, secondary batteries using an alkali metal such as lithium have been attracting public attention. However, when lithium metal alone is used in a negative electrode of the battery, there is a problem that, as a result of repeated charges and discharges (i.e. repeated depositions and dissolutions of lithium metal), dendrites (crystals in a shape of branches of a tree) are generated on the surface of the metal and, as they grow up, they penetrate through a separator of the battery and contact the positive electrode whereby a short circuit is induced in the inner part of the battery. It has been known that, when a lithium alloy is used as a negative electrode of the second battery instead of a lithium metal, formation of the dendrite is prevented as compared with the case of the use of a lithium metal alone and the characteristics of the charge-discharge cycle are improved. However, even the use of the alloy is not effective in perfectly preventing the formation of the dendrite but a considerable possibility of a short circuit in the inner part of the battery still remains. In addition, the use of the alloyed negative electrode causes an increase in the weight whereby an advantage of the light weight of the secondary batteries by the use of lithium is deteriorated.
In recent years, there has been a development on the matrix materials such as electroconductive polymers and carbon materials utilizing the absorption-desorption steps of lithium ion instead of utilization of lithium metal or alloy thereof for negative electrode. As a result thereof, formation of dendrites which occurred when lithium metal or alloy thereof is utilized does not take place on a principal basis whereby a problem of short circuit in the inner part of batteries has been greatly reduced. It has been especially known that the absorption-desorption potential of the carbon materials is nearer the deposition-dissolution potential of lithium than other materials. Among them, a graphite material is theoretically capable of incorporating one lithium atom per six carbon atoms into its crystal lattice and, therefore, it is a carbon material having a high capacity per unit weight and unit volume. In addition, its intercalation-deintercalation potential of lithium is flat or uniform and it is a chemically stable material and, accordingly, it greatly contributes to the cycle stability of the battery.
Examples are the use of the carbon material of a graphite type as an active material for the negative electrode as disclosed in J. Electrochem. Soc., Vol. 137, 2009 (1990) and the Laid-Open Japanese Patent Laid-Open Nos. 4(1992)-115,457, 4(1992)-115,458, 4(1992)-237,971, etc. and also the use of surface-processed carbon material of a graphite type as an active material for the negative electrode as disclosed in the Japanese Patent Laid-Open Nos. 4(1992)-368,778, 5(1993)-28,996 and 5(1993)-114,421.
As mentioned above, the material of a graphite type affords a discharge capacity which is nearly the same as the theoretical capacity in an organic electrolytic solution mainly consisting of ethylene carbonate (EC). In addition, its potential in a charge--discharge cycle is slightly higher than the potential in a dissolution--deposition of lithium and is very uniform whereby, when a battery which is prepared using the carbon material of a graphite type is used as an active material for the negative electrode, the battery having a high capacity and also a highly uniform battery voltage can be materialized.
Although the carbon material can achieve a high capacity as mentioned above, there is still a problem that, due to its high crystallinity, it causes a decomposition of an electrolytic solution (a nonaqueous ionic conductor). For example, propylene carbonate (PC) which is one of the solvents for organic electrolytic solutions has been widely used as a solvent for the electrolytic solution for lithium batteries because of its wide potential range, low freezing point (-48.8.degree. C.) and high chemical stability. However, it was reported in J. Electrochem. Soc., Vol. 142, 1746 (1995) that, when the carbon material of a graphite type is used as a negative electrode active material, the negative electrode consisting of a carbon material of a graphite type is not capable of being charged and discharged in case PC of as little as 10% is present in the electrolytic solution.
It has been widely known that a carbon material of a graphite type can be used as a negative electrode for lithium secondary batteries only when an electrolytic solution of a mixed solvent type consisting of an EC and a solvent having a low viscosity is used. However, an electrolytic solution mainly comprising an EC has a low ionic conductivity at low temperatures and, when a secondary battery using said electrolytic solution and a carbon material of a graphite type as a negative electrode is prepared, it is very difficult to improve the temperature characteristics or the current characteristics of said battery by means of selection of the electrolytic solutions because the choices for the solvents which can be used for secondary batteries are very little.
In order to solve such problems, the use of carbon materials wherein the surface of graphite particles are coated with a low crystalline carbon as negative electrode active materials for secondary batteries has been proposed as mentioned, for example, in the Japanese Patent Laid-Open Nos. 4(1992)-368,778 and 5(1993)-121,066. That is an effective means for inhibiting the decomposition of the electrolytic solution to increase the discharging capacity and to improve the cycle characteristics. However, when a secondary battery is prepared using an electrolytic solution mainly comprising a PC, there are problems that, as a result of pulverization for making the particle size uniform during the manufacturing stage of its negative electrode or as a result of kneading upon manufacture of the electrode materials and of coating onto a power collecting plate, the lowly crystalline carbon coated on the graphite particle surface is peeled off whereby the electrode is destroyed by generation of gas due to decomposition of the electrolytic solution resulting in a decrease in a capacity of the battery and a deterioration of the cycle characteristics. In addition, further steps such as pulverization are necessary whereby there is another problem that the manufacturing cost is high even if a graphite material with a low price is used.
As a method where far lower manufacturing cost can be expected, there is a method in which a carbon precursor such as pitch is mixed with the graphite followed by calcinating as disclosed in the Japanese Patent Laid-Open No. 6(1994)-84,516. However, there is a problem in this method that, because the liquid-phase steps are used, the graphite particles coated with a lowly crystalline carbon adhere each other and active surfaces -of the graphite appear again by a pulverization in the manufacturing steps of the negative electrode whereby decomposition of the PC takes place.
As mentioned hereinabove, a problem has now been found that, when the surface of the graphite particles are coated with a low crystalline carbon, the adhesive strength of the graphite particles with the low crystalline carbon is weak resulting in an immediate peeling off whereby decomposition of the electrolytic solution is resulted. Accordingly, there is a problem even by such a method that the characteristic properties of the battery are deteriorated and the yield in the manufacture of batteries is lowered.